This invention relates to radio frequency communications systems, and more particularly to an adjustable radio frequency antenna circuit and method of adjusting sensitivity.
As antennas are implemented on an ever-broader range of communications devices, there is a continuous need to provide antenna implementations that realize a desired signal sensitivity, both in transmitting and receiving modes. Particularly, for radio frequency tag devices there is a need to provide a tag device having antennas with adjustable reception for tracking a variety of objects including goods, items, persons, or animals, as well as any moving or stationary and animate or inanimate object. One way is to produce a large assortment of tag devices, each having an antenna with a uniquely constructed radio frequency transmitting and receiving sensitivity. However, once produced each device will have an antenna with a single built-in sensitivity, and without the implementation of additional circuitry or modifications to the antenna, the antenna sensitivity can not be modified. Therefore, there is a need for an antenna construction having an easily adjusted receiver sensitivity. Likewise, there is a similar need for an antenna construction having an easily adjusted transmitter sensitivity.
Another reason for implementing an adjustable radio frequency communications device is readily apparent in the field of electronic identification systems in order to extend useful life where battery powered devices are utilized. For example, battery powered transponders or tag devices are presently commercially available that are affixed to an object in order to monitor location of the object. A controller or interrogator unit transmits an interrogation signal to the device during use in order to initiate location of the device. The device receives the signal, and generates and then transmits a responsive signal. The interrogation signal and the responsive signal are typically radio frequency (RF) signals produced by an RF transmitter circuit. Due to the relatively large transmission distances available with RF signals, RF-based transponder devices tend to be more suitable for applications requiring tracking of a tagged device that may not be in close proximity to an interrogator unit. However, using a large number of these devices, the interrogator unit tends to trigger frequent wake-up of each device. As a result, responsive signals are frequently generated. For each case of a battery-powered device, the life of the battery is severely diminished due to frequent unintentional wake-ups of the device. Therefore, there is a need to produce tag devices having different receiver sensitivities, and to produce tag devices having either factory or user adjustable antenna circuit sensitivity. Such constructions comprise the subject matter of this invention.